var xmlhttp;

function doAjax(url, parametros)
{
	xmlhttp=PostXmlHttpObject();
	
	if (xmlhttp==null)
	{
		alert ("Browser does not support HTTP Request");
		return;
	}
	
	xmlhttp.onreadystatechange=stateChanged;
	xmlhttp.open("POST",url,true);
	xmlhttp.setRequestHeader("Content-Type", "application/x-www-form-urlencoded; charset=UTF-8");	
	xmlhttp.send(parametros);
}

function stateChanged()
{
	if (xmlhttp.readyState==4)
	{
		if ( xmlhttp.status==200 )
		{
			//declarado no javascript da tela
			trataResposta();
		}
	}
}

function PostXmlHttpObject()
{
	if (window.XMLHttpRequest)
	{
		// code for IE7+, Firefox, Chrome, Opera, Safari
		return new XMLHttpRequest();
	}
	if (window.ActiveXObject)
	{
		// code for IE6, IE5
		return new ActiveXObject("Microsoft.XMLHTTP");
	}
	
	return null;
}